The MenuItem.ToggleSizeAllocated event utilizes this delegate:

Event data is passed via the Gtk.ToggleSizeAllocatedArgs parameter.

To attach a Gtk.ToggleSizeAllocatedHandler to an event, add the ToggleSizeAllocatedHandler instance to the event. The methods referenced by the ToggleSizeAllocatedHandler instance are invoked whenever the event is raised, until the ToggleSizeAllocatedHandler is removed from the event.